Reverse drag flow power node

Reverse drag flow production makes it possible to configure multiple productions in series, without any switching valves but hydraulic separated by an open header. The flow in the open header can go in both directions, and is fully dependent on the designed flow rates, similar to the way a low loss header can have it’s mixing point either on supply and return. The following figure shows these base circuits for connection both on supply and return. On the right the parameter window that pops up when you click on the base circuit is shown.

Open

Compared to serial and parallel, the shunt connection requires more parameterisation due to the hydraulic separation. Instead of the temperatures for both gates being defined through the propagated temperatures, the mixing makes it possible to arrive with a range of temperatures. This temperature is what is called the draw flow power. To explain this, let’s look at the following example where a heat pump is placed in primary and a boiler is used as shunt in the supply. There is a regular power propagation of 100 kW meaning the full flow (21,74 m³/hr) goes over the heat pump. Q=mcdT therefore defines the outgoing temperature, which is 44°C. So far, everything is the same as a standard serial connection. The difference now stems in the entering temperature. The drag flow power is defines as 90°C, meaning the boiler will supply 90°C to the shunt connection, which defines the mixing point at this connection. Therefore the 44°C is propagated to the boiler (due to the dividing point which now exists here. Using Q=mcdT, the software can now calculate the flow (due to the high dT, the flow is small).

Open

Let’s now say that instead of a boiler, there is a high temperature heat pump used, bringing the temperature to 65°C. Every part of the propagation remains exactly the same, apart from the draw flow power which has to be adjusted to the 65°C, and therefore, due to Q=mcdT, the flow increases.

The formulas behind this base circuit are the following:

The parameter window has three different parameters that determine the operation of this base circuit. Namely ‘Drag flow power’, ‘Drag flow power percentage’ and ‘Power propagation’.

* The three options are: ‘Regular’, ‘Primary full load’ and ‘Both full load’

** Be careful in filling in this value, see example 1 for more information

Examples

All these examples use the same installation with an equivalent radiator capacity of 200 kW. The only thing that changes are the parameters on the reverse drag flow base circuit.

The different values for ‘Drag flow temperature’ are filled in for this first example.

As you read above, the ‘Drag flow temperature’ parameter has an important influence on the design flows in the sytem. This is better shown by a first example.

Both the left and right model have a ‘Drag flow power’ parameter that is set to 120 kW. This power is propagated further upstream the drag flow gate. The power of the primary gate is subsequently calculated as the total power, (in this case 200 kW), minus the drag flow power, resulting in a power propagation of 80 kW further upstream the primary gate.

Next, the temperature regime, coming from the units downstream the hybrid block, is used to determine the design supply & return temperatures on both the primary & drag gate. For the primary gate, the supply temperature is set equal to the propagated supply temperature downstream the hybrid block (in this case 60 °C). The return temperature is determinated by taking the same ratio between the temperature difference on the primary gate & the temperature difference on the gate going downstream as the ratio of their power progagation. (In this case is the return temperature 52 °C, such that the temperature difference of 8 °C at the primary gate and 20 °C at the gate going downstream have the same ratio as their power propagation of 80 kW & 200 kW, respectively.)

On the left model, the ‘Drag flow temperature’ parameter has a lower supply temperature than the calculated primary return temperature. The Optimiser will therefore link the parameter to the return temperature of the drag flow gate. The design supply temperature of the drag flow gate will subsequently be taken equal to the return temperature of the primary gate.

On the right model, the ‘Drag flow temperature’ parameter has a higher supply temperature than the calculated primary return temperature. The Optimiser will therefore link the parameter to the supply temperature of the drag flow gate. The design return temperature of the drag flow gate will subsequently be taken equal to the return temperature of the gate downstream the hybrid block.

Finally, the design flow rates at both the primary and drag flow gate are calculated using the design power and the temperature difference (see formula above). Always double-check the flow rate at the drag flow gate!

The following figures show what could happen when you don't double-check the flow rate.

On the left model, the return temperature value is filled in the ‘Drag flow temperature’. As you can see, the flow rate is absurdly high. On the right model, a ‘Drag flow temperature’ of only 1° lower than the return temperature is filled in which makes the flow rate also too high. The high return temperature can only be explained by the following figure. Most of the flow follows the dark red arrow which will end up being disastrous for the system's efficiency.

For the second example, the different values for ‘power propagation’ are filled in.

When the reverse drag flow base circuit is set on ‘Regular’, the heat flow that is filled in at ‘drag flow power’ is filled in on the drag flow gate and the rest, 200kW minus 120kW, is filled in on the primary gate. When the base circuit is set on ‘Primary full load', the full power is set on the primary gate and everything before the primary gate is sized on the full load. The drag flow gate and everything before it is sized on the value that is filled in at 'drag flow power’. When the base circuit is set on ‘Both full load', the full power is set on both gates and everything before these gates is sized on the full load.

Common errors

Drag flow power node

The principle is the same as the reverse drag flow power node, the only difference is that the flow inside the open header can only go in one way because of the hydraulic connections. The following figure shows on the left the base circuits of reverse drag flow on supply (top base circuit) and on return (lower base circuit). On the right the parameter window that pops up when you click on the base circuit of reverse drag flow on return. The gate on the left is the primary gate and the gate perpendicular to the other pipes is the drag flow gate.

The formulas behind this base circuit are the following:

The parameter window has three different parameters that determine the operation of this base circuit. Namely ‘Drag flow power’, ‘Drag flow power percentage’ and ‘Power propagation’.

* This parameter is optional and does not need to be filled in, however, if this is filled in

** When you fill in 100%, you will get an error (see common errors a bit down). It is advised to fill in ‘Full load on primary gate’ on the parameter power propagation.

*** This division is necessary due to the calculation method Hysopt uses, which flows from the end units down to the production units. This division between multiple pipes requires parameterisation through the hybrid blocks.

**** The three options are: ‘Regular’, ‘Primary full load’ and ‘Both full load’. See example 2 for more info about the effect of each option.

The following examples show how each of these parameters impacts the operation of this base circuit.

Examples

For this first example, the different values for ‘Drag flow temperature’ are filled in.

Both base circuits have the same ‘Drag flow power’ but different values for ‘Drag flow temperature’. This value defines the drag flow supply temperature. The explanation is almost entirely the same as in example 1 of ‘Reverse drag flow node’ (see above). The only difference is that the filled-in ‘Drag flow temperature’ is always for the supply temperature of the drag flow gate. Always double-check the flow rate of the drag flow circuit! When the temperature difference is set to small, the flow rates reach an unrealistic high value.

For the second example, the different values for ‘power propagation’ are filled in.

When the drag flow base circuit is set on ‘Regular’, the heat flow that is filled in at ‘drag flow power’ is filled in on the drag flow gate and the rest, 200kW minus 120kW, is filled in on the primary gate. When the base circuit is set on ‘Primary full load', the full power is set on the primary gate and everything before the primary gate is sized on the full load. The drag flow gate and everything before it is sized on the value that is filled in at 'drag flow power’. When the base circuit is set on ‘Both full load', the full power is set on both gates and everything before these gates is sized on the full load.

Common errors